The present invention relates to an integrated device with trimming elements.
In integrated devices of various types there are circuit elements of which a characteristic parameter such as, for example, resistance, has to have a predetermined tolerance; typically, these circuit elements are used to generate a reference quantity (a voltage or a current) and the tolerance must therefore be very low to ensure that the reference quantity has a sufficiently precise and stable value. However, a very low tolerance value is difficult to achieve because of accuracy limits inherent in the processes by which the integrated devices are produced (machine inaccuracy, inaccuracy of photolithography and etching processes, and the like).
This problem is generally solved by a subsequent trimming process of the circuit elements. In particular, various trimming elements provided in the integrated device can be connected to the circuit element selectively in order to correct an inaccuracy thereof. During the trimming process, the characteristic parameter of the circuit element is measured and compared with a nominal value thereof. The trimming elements are activated or deactivated according to the result of this comparison so as to achieve the required tolerance.
A prior art solution, known as xe2x80x9cmetal zapping,xe2x80x9d consists of the use of metal fuses. The circuit element is formed by several elemental components connected to one another, for example, in series; a trimming element in series with a metal fuse is connected in parallel with each elemental component. The terminals of each metal fuse are accessible from outside the integrated device by means of a pair of firing pads. If a current of a high value (for example, 300 mA) is supplied to the firing pads, the metal fuse is blown; the corresponding trimming element is thus deactivated (since it is disconnected from the elemental component).
A disadvantage of this solution is that, in order to be blown, the metal fuses must not be covered by a protective layer. Any external agents may consequently create a short-circuit in the region of a blown metal fuse or may corrode a metal fuse which has not blown; this undesirably modifies the characteristic parameter of the circuit element, rendering the integrated device unreliable.
A different prior art solution, known as xe2x80x9cZener zapping,xe2x80x9d consists of the replacement of the metal fuses with Zener diodes. If a current of high value is supplied to the firing pads, the Zener diode is blown and is brought to a breakdown condition in which it behaves as a short-circuit; the corresponding trimming element is thus activated (since it is connected to the elemental component).
The results of the trimming process thus achieved, however, are greatly dependent on the manufacturing process and on the geometry of the Zener diodes; this requires the characteristics of these Zener diodes to be defined extremely carefully.
In both of the above-mentioned solutions, the firing pads take up a considerable amount of space on a semiconductor chip in which the integrated device is formed; this leads to the need to limit the number of firing pads, and hence trimming elements, correspondingly limiting the degree of accuracy which can be achieved.
Moreover, both the metal zapping technique and the Zener zapping technique are generally applied to a wafer of semiconductor material on which the same integrated device is simultaneously formed in several identical areas. The use of these solutions in an integrated device encapsulated in a package in fact requires too great a number of external pins (connected to the firing pads) and metal connection tracks which are very wide in order to be able to withstand the current necessary to blow the metal fuses or the Zener diodes. The trimming process consequently cannot correct variations in the characteristic parameter of the circuit element caused by mechanical and thermal stresses undergone by the integrated device during an encapsulation operation.
An embodiment of the present invention overcomes the above-mentioned drawbacks. The embodiment provides an integrated device comprising a circuit element and a plurality of trimming elements which are connectable selectively to the circuit element for achieving a predetermined tolerance of a characteristic parameter of the circuit element. The integrated device includes a plurality of electronic switches each switchable between a first state and a second state that respectively activates and deactivates a corresponding one of the trimming elements, and means for storing an indication of the states of the electronic switches and for operating each electronic switch in the first state or in the second state, in accordance with the indication stored.
Moreover, the present invention also proposes a corresponding method of producing an integrated device.